1. Field of the Invention
The present invention relates to a display device using a display panel. In particular, the present invention relates to a display device having a structure in which a translucent substrate including a light shielding portion formed on a periphery of a display surface side is bonded to a display panel.
2. Description of the Related Art
A liquid crystal display device (LC display device) is used in various devices such as a cellular phone, a personal digital assistant (PDA), an electronic dictionary, a car navigation system, and a music player. FIGS. 6A and 6B illustrate a conventionally-known liquid crystal display device. FIG. 6A is a plan view and FIG. 6B is a longitudinal sectional view. As illustrated in FIGS. 6A and 6B, the liquid crystal display device includes a liquid crystal display panel (LCD) 62 and a translucent substrate 68 thereabove, which serves as a front plate. In the LCD 62, a liquid crystal layer is sandwiched between two glass substrates, and polarizing plates are bonded to outer surfaces of the two glass substrates. The translucent substrate 68 is entirely transparent, and a black light shielding portion 74 shields light of an outer peripheral portion of the LCD 62 and defines a display region of the LCD 62. The light shielding portion 74 is provided in order that an image beyond the display region may not be viewed by a user and that design aesthetics of a display portion may be improved.
In such a display device as described above, a glass substrate forming the translucent substrate 68 or the LCD 62 is made thinner as the various devices become thinner (for example, thickness of 0.20 mm to 0.25 mm). Further, there are increasing demands for narrowing a gap between the LCD and the translucent substrate (for example, gap equal to or smaller than 0.2 mm).
However, along with the achievement of the thinner liquid crystal display devices, the LCD 62 has become more fragile due to dropping of portable devices onto which the liquid crystal display device is mounted or due to objects fallen onto a display surface of the LCD 62. In addition, there has been a risk that, when an air space is interposed between the LCD 62 and the translucent substrate 68, light may be reflected on the display surface of the LCD 62 or a lower surface of the translucent substrate 68, and hence the display surface becomes dark due to this reflection loss.
For example, JP 09-274536 A discloses the method of filling a gap between the LCD 62 and the translucent substrate 68 with a transparent adhesive. FIG. 7 is a longitudinal sectional view illustrating an end portion of the conventionally-known liquid crystal display device. A liquid crystal display device 60 has the structure in which the gap between the LCD 62 and the translucent substrate 68 which serves as the front plate is filled with a transparent adhesive 76. In the LCD 62, a thin film transistor (TFT) substrate 92 onto which TFTs are mounted and a color filter substrate 96 including color filters 79R, 79G, and 79B formed therein are bonded to each other through a sealing agent 94, and a liquid crystal layer 90 is sandwiched between both the substrates. A transparent electrode is formed on a liquid crystal layer 90 side of the color filter substrate 96. A black matrix 78 is provided in the color filter substrate 96. Polarizing plates 65 and 67 are bonded to outer surfaces of the TFT substrate 92 and the color filter substrate 96, respectively. With this structure, the LCD 62 and the translucent substrate 68 are integrated with each other through the transparent adhesive 76, whereby shock resistance can be increased. Further, with the use of the transparent adhesive 76 having a refractive index which is approximate to that of the translucent substrate 68 or the polarizing plate 65 (67), the reflection loss generated at an interface between the polarizing plate 67 and the transparent adhesive 76 or between the translucent substrate 68 and the transparent adhesive 76 may be reduced.
As the transparent adhesive 76, an optical adhesive of a photo curable type, which is cured by light such as ultraviolet rays or visible light, is mainly used. A heat curing adhesive which is cured by heat may be used, but is difficult to use because, for example, liquid crystals or polarizing plates are deteriorated when the heat curing adhesive is exposed to a high temperature equal to or higher than 100 degrees, or shelf life of the adhesive is short.
On the other hand, a photo-curable adhesive is convenient, for example, it can be bonded under environment of room temperature. A step of curing the photo-curable adhesive as described above includes a first step of filling the gap between the LCD 62 and the translucent substrate 68 with an transparent adhesive 76 and then irradiating the transparent adhesive 76 with light such as ultraviolet rays from a translucent substrate 68 side, and a second step of irradiating the transparent adhesive 76 with light from a lateral side of the LCD 62. Specifically, in the first step, light A1 is emitted from the translucent substrate 68 side to cure the translucent adhesive 76 which is positioned at a translucent portion of the translucent substrate 68. However, the transparent adhesive 76 positioned under the black light shielding portion 74 is shaded, which causes insufficient curing thereof. For this reason, in the second step, light A2 is emitted from the lateral side of the LCD 62. As a result, the transparent adhesive 76 positioned under the light shielding portion 74 is cured.
However, in the liquid crystal display device described above, the gap between the LCD 62 and the translucent substrate 68 is small, and therefore the light A2 is unlikely to penetrate therethrough. In addition, light absorption in the light shielding portion 74 of the translucent substrate 68 is large, and the light A2 is propagated while being repeatedly reflected on the light shielding portion 74 to be attenuated, which involves a risk that the transparent adhesive 76 positioned in the vicinity of the light shielding portion 74 may be insufficiently cured.
Further, the LCD 62 is also provided with the black matrix 78 so as to be opposed to the light shielding portion 74 of the translucent substrate 68. For this reason, light is also absorbed in the black matrix 78, whereby the light A2 is further attenuated. As a result, an uncured portion of the transparent adhesive 76 is left in a lower region of the light shielding portion 74.
When an uncured portion is left in the transparent adhesive 76, sufficient bonding strength cannot be obtained in the vicinity of the LCD 62 and the light shielding portion 74 of the translucent substrate 68, which involves a risk that mechanical strength such as strength against falling may be impaired.
Further, in the transparent adhesive 76, a volume of the adhesive itself contracts during curing. When an uncured portion is left in the transparent adhesive 76, an amount of the volume contraction is small in the vicinity of the light shielding portion 74. Therefore, there is a risk that a thickness of the transparent adhesive 76 may become uneven and the LCD 62 to which the translucent substrate 68 is bonded may be deformed, which is specifically described with reference to FIG. 8. FIG. 8 is an explanatory view of the conventionally-known liquid crystal display device, in which an upper stage is a plan view thereof, a middle stage is a longitudinal sectional view thereof, and a lower stage illustrates a gap between the TFT substrate 92 and the color filter substrate 96. As illustrated in FIG. 8, the light shielding portion 74 for shielding light is provided on a periphery of the translucent substrate 68. The translucent substrate 68 serving as a front plate and the LCD 62 are bonded to each other through the transparent adhesive 76. In the LCD 62, the TFT substrate 92 and the color filter substrate 96 are opposed to each other through the sealing agent 94 or a spacer member 98, and the liquid crystal layer 90 is formed therebetween. When an uncured portion is left in the transparent adhesive 76 positioned under the light shielding portion 74, an amount of the volume contraction becomes small in the vicinity of the light shielding portion 74 and becomes large in the display region. As a result, the TFT substrate 92 and the color filter substrate 96 are deformed to thereby cause fluctuations of a gap (cell gap) between the TFT substrate 92 and the color filter substrate 96. Hence, there is a fear of deterioration of image quality in the display surface, such as display fluctuation. Particularly, a thickness of a substrate forming the LCD 62 is conventionally set to about 0.5 mm, whereas, in recent years, the thickness has been set to 0.2 mm to 0.25 mm by abrasion and thinning through etching or polishing. Accordingly, the LCD 62 is particularly likely to be affected by unevenness of the volume contraction.
In response thereto, the film thickness of the light shielding portion 74 may be reduced to thereby reduce optical density and enhance transmittance from the translucent substrate side, or the light shielding portion may be configured to be a half mirror. However, when a black concentration is reduced, the end portion of the LCD 62 may be viewed from thereabove or light emitted from a backlight may be leaked from therebelow upwardly, which results in poor design aesthetics. Accordingly, the black concentration of the light shielding portion 74 is preferred to be high.